Astronomers Discover the Nearest Young Planet-Forming Star

A team of scientists including University of Hawaii astronomers have found the nearest example of a young planet-forming star.

Using the telescopes on Mauna Kea, Hawaii, they have discovered a spectacular circumstellar dust disk with indirect evidence for newly formed planets around the star AU Microscopium (AU Mic). At a distance of only 33 light years, this is the nearest star with a visible disk, enabling astronomers to directly observe the primordial material for making planets.

"We know that extrasolar planets are common, but understanding how they form is an outstanding question. Because AU Mic is so near to Earth, it provides us a special opportunity to examine planet formation in great detail," said team member Dr. Michael Liu, an astronomer at the University of Hawaii Institute for Astronomy.

The results will be published in this week's online Science Express , the March print edition of Science, and an upcoming issue of the Astrophysical Journal. The authors are Drs. Michael C. Liu and Jonathan P. Williams of the University of Hawaii, and Drs. Paul G. Kalas and Brenda C. Matthews of the University of California, Berkeley.

AU Mic is a dim red star, with only half the mass and one-tenth the energy output as the Sun. Previous studies had shown that AU Mic is about 12 million years old. In comparison, our Sun is about 4.6 billion years old.

"Unfortunately, we can't go back in time and observe our own Solar System. But by studying these very young stars, we can examine how planets are forming around them, and thus indirectly learn about the origin of our own Solar System," said Liu.

The team used the James Clerk Maxwell Telescope on Mauna Kea, Hawaii to study the sub-millimeter radiation from AU Mic. This radiation, which is invisible to the naked eye, comes from very cold dust grains orbiting the star in the form of a disk. Such disks are believed to be the nurseries for forming planets.

By analyzing the radiation, the team deduced that dust particles only existed at large distances from the star, and were missing inside a radius of about 17 AU. This would be slightly inside the orbit of Uranus in our own Solar System.

"The dust missing from the inner regions of AU Mic is the telltale sign of an orbiting planet. The planet sweeps away any dust in the inner regions, keeping the dust in the outer region at bay," said Liu.

The team's visible light images of AU Mic reveal a spectacular edge-on disk, extending out to at least 210 Astronomical Units (AU), or about 20 billion miles. In comparison, the known edge of our Solar System is about 50 AU, or four times smaller. AU Mic's disk is visible due to small dust particles which reflect the light of the central star.

The images were obtained with an instrument known as a coronagraph deployed on the with the University of Hawaii's 2.2-meter telescope. The coronagraph blocked out the bright glare of the central star, allowing detection of the faint edge-on disk.

"This fascinating system shows how the exceptional clarity, darkness, and transparency of the Mauna Kea skies allows Hawaii astronomers to make frontier discoveries," said Dr. Rolf Kudritzki, Director of the Institute for Astronomy.

"AU Mic is a common red dwarf star, which comprise 85% for all stars. By studying this nearby system, we might learn about how the majority of planetary systems can form," said team member Dr. Paul Kalas, an astronomer at the University of California, Berkeley and a Ph.D. recipient from the University of Hawaii.

Images of disks around nearby stars are very rare, and AU Mic is the closest dust disk found since the discovery 20 years ago of a disk around beta Pictoris, a star about 2.5 times the mass of the sun and 65 light years away. Though the two stars are far apart on the sky, they appear to have been formed at the same time and are traveling together through the galaxy.

AU Mic is close enough that future imaging with the Hubble Space Telescope or ground-based telescopes using adaptive optics can study the detailed structure of the disk and perhaps directly image the light from any planets.

"We're waiting for the next observing season to go back and study the physical properties of the disk. But we expect other teams to do the same thing - there will be lots of follow-up," said Kalas.

"Astronomers will be studying AU Mic for many years to come," said Liu.

This work was supported by the National Science Foundation and the NASA Astronomical Search for Origins program.

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. Refer to http://www.ifa.hawaii.edu/for more information about the Institute.